The lymphatic system and the cardiovascular system are closely related structures that are joined by a capillary system. The lymphatic system is important to the body's defense mechanisms by filtering out organisms that cause disease and producing lymphocytes that attack foreign organisms and generate antibodies. It is also important for the distribution of fluids and nutrients in the body, because it drains excess fluids and protein from interstitial tissues. Lymph is the fluid that seeps outside the blood vessels in interstitial spaces of body tissues and is then absorbed by lymphatic capillaries to flow back into the bloodstream through the lymphatic vessels. The terminal structures of the lymphatic vessels include the right lymphatic duct, which drains lymph fluid from the upper right quarter of the body above the diaphragm and down the midline, and the thoracic duct, located in the mediastinum of the pleural cavity which drains the rest of the body. Through the flow of blood in and out of arteries, into the veins, and through the lymph vessels and nodes, the body is able to eliminate the products of cellular breakdown and bacterial invasion.